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Moisture content of wood and building boxes

I've just got a bunch of wood from a mill to build about 40 boxes, and I'm wondering about drying - and how that will affect my measurements.

Wood shrinks as it dries, so I've heard that's where the 9 5/8" deep dimension came from - manufacturers building boxes out of not-so-dry wood and allowing for shrinking as it dries - the box stays safely withing bee space requirements. 9 5/8 is about an 8th too big for proper bee space.

So do I need to reach a specific dryness before making final cuts and assembling? Or is it safe to take wetish wood and build to 9 5/8 dimensions?

Re: Moisture content of wood and building boxes

Well, I don't think that you have to care much about wood movement so much. If all the the wood has the
same/similar moisture content you should be fine.
My only concern would be if the wood is very wet, gluing could cause some trouble. Really wet surface just don't glue
very well.

Re: Moisture content of wood and building boxes

Re: Moisture content of wood and building boxes

Adam Foster Collins.... I use pine, cypress, and poplar rough saw from saw mills. My experience is that a 10 1/2 inch board will shrink between 1/2 and 3/4 inch over a period of one year If you have green, rough cut, or as you put it, wetish wood, and you cut it to 9 5/8 you would be looking at a finshed board, after shrink, of about 9 inches. I would say unacceptable.

I try for 12 - 18 % moisture. This normally takes 9 months to 1 year, stripped, and air dried.

There is another good thread on this same topic in Equipment and Hardware review.

Re: Moisture content of wood and building boxes

Originally Posted by Michael Palmer

Is the wood kiln dried, air dried, or green?

I believe the wood has been aired dried for some time. I doubt the mill would mill and sell green wood, but I guess it's possible. The wood is dressed, but there are a few boards where you can see it was the end of the log, and those ends are well-weathered and grey from what appears to be a considerable time outside.

I don't have a moisture meter, but I suppose I can do the "weigh-sample-dry-it-in-the-oven-and re-weigh" approach to finding out what it's moisture content is, and then go to the percentages Cleo suggests...

Re: Moisture content of wood and building boxes

I built 10's of boxes this year out of hemlock that was only cut for several days. I used rebate joints instead of box/finger joints so that shrinking didn't matter. I had no trouble at all. I made sure that I had planed, cut, built and painted within 3 days though - in hopes the layers of paint would slow down the drying of the wood and keep it more stable. It worked great. No issues at all. I did not measure my boxes after a few months but they may be a bit more shallow. I know frame clearance between boxes is tight on some of them. Maybe add an extra 1/8 or 1/4 to your depth just in case. It won't be enough to matter if they don't shrink.

Re: Moisture content of wood and building boxes

Originally Posted by Adam Foster Collins

I believe the wood has been aired dried for some time. I doubt the mill would mill and sell green wood,

I would doubt it too Adam. Cut off a 6 inch piece, measure across the width put it in an oven @250 for a couple of hours, measure again. See how much it shrinks. Keep in mind it would be easier to plane off 1/8 inch than to scab on 1/8 inch.

Re: Moisture content of wood and building boxes

Green lumber from a mill is actually a common practice, it takes a lot of space/time or a kiln to supply dry material..and you normally pay for that additional step. logs lying in the yard rarely drop below fiber saturation point (the moisture level at which the wood begins to shrink) The mills intentionally keep the logs damp and saw them green; it is easier on the equipment and avoids/reduces losses to end checking.

That said, if the material has been surfaced it is likely to have been at least air dried down to 14% which would be fine for outdoor work. It is rare and a bit pointless to run true green lumber through a planer.

Mike is right, just ask the mill, they should have a reasonable idea of the material's moisture content.

Ace, your test doesn't tell you anything useful except that the wood is not at 0% if there is shrinkage. The oven test is done with weight of the wood and then carried to "oven dry" or 0%MC (the point at which the sample stops losing mass) the loss in water weight relative to the weight of the dry wood then allows you to calculate the original MC of the sample. If you don't know your starting point or end points, the amount of shrinkage you see is meaningless. Once you know the MC of the material it is a simple matter to calculate the shrinkage by species and cut that the boards will reach at environmental equilibrium.

If the wood has already been planed to thickness, then Mike's point about cupping certainly comes into play...that's one reason it's best to dress lumber that is at equilibrium with it's intended use...it eliminates those stresses by removing whatever cupping has occurred as it dries to that point. If it is dressed and flat now...I would use it now

Boxes the size of hives are small enough and thin enough to self restrain mild cupping and warping forces. If you know your MC you can likely just cut your material to account for the anticipated shrinkage across the face of the board....not best practice but certainly doable.

Re: Moisture content of wood and building boxes

Ace,
Just thought about it more your suggestion could work if you dried the board untill shrinkage stopped (meaning 0% MC) and then used the dimensional change and existing shrinkage tables to back calculate the original MC. I have never heard of doing it that way but it should work. Unfortunately shrinkage is non linear so it would be somewhat inaccurate.

Re: Moisture content of wood and building boxes

Originally Posted by windfall

Ace, your test doesn't tell you anything useful except that the wood is not at 0% if there is shrinkage.

The amount of shrinkage from 14% to oven dry will be considerably less than green to oven dry. The rate at which the moisture leaves the wood decreases as the moisture content decreases. I don't know if 250 degrees for a couple of hours in a dry oven would bring the sample to 0% if the start point was 14%. I would rather doubt it. What do you think?

Re: Moisture content of wood and building boxes

my mistake, shrinkage is linear. I was thinking of some strength properties relative to MC.

A couple hours...I doubt it. obviously dependent on species and thickness. You could get there faster with a microwave..... but really I like my moisture meter if you work regularly in wood it is 50$ that instantly eliminates all sorts of questions and guesswork.

Re: Moisture content of wood and building boxes

Originally Posted by windfall

but really I like my moisture meter if you work regularly in wood it is 50$ that instantly eliminates all sorts of questions and guesswork.

Does it? I am guessing it really only probes the surface. The surface could easily be much different then the rest of the board if the board is not in a controlled environment for a considerable amount of time.
I have used IR scanners that could only read the surface and they were 20K. A nuclear mass scanner can penetrate much deeper into a substrate but they are upwards of 100K. I can't see a $50. instrument telling you much under varying conditions. I think you would have to be very observant on controlling the variables.

Re: Moisture content of wood and building boxes

I built a bunch of boxes years ago from a local mill, and the wood was air dried for about a year. After the wood is worked into boxes, it will shrink, and I found the boxes that I cut 1/8 over size shrunk to being 1/8 to small in every direction. I would not worry about it too much, as these boxes will prove their purpose for the next few years.
which ever way you look at it, these boxes will be a pain, you are going to cut them 1/8 larger than needed to allow for shrinkage, so your bee space is wrong, then they will eventually shrink , to the right size , and many will over shrink and cause you bee space problem

thats the nature of home made boxes

the guy that custom builds boxes here will measure every board before cutting into boxes and frames. quality and consistency is very important and that is why it is sometimes worth the extra money to buy them

Re: Moisture content of wood and building boxes

Ian,
I can't say what you may have experienced or why. 1" softwood lumber properly air dried for a year should be at EMC with outdoor conditions....that is just what you want for hives or any woodwork in an exterior application. Around here that averages 12% MC and that is what my hives read after several years of use. Kiln dried material (6-8%) is going to expand and get bigger unless you live somewhere very dry. And I really don't know how the hive could shrink "in every direction"...wood movement along it's length is negligible. It sounds like the wood you used was still pretty wet.

Ace, pin meters are the industry standard. They work very well when used properly. Admittedly the 50$ units are less accurate than the 500$ but even the 50$ one is really quite useful. Pin penetration can be from 1/16" to 1"+ depending on probe type used. If your local conditions have changed radically over the last few days, the material is very thick, or it just came out of a kiln and you need an accurate reading. You simply cut 2" off the end of the board and stick the pins in the "middle".

Re: Moisture content of wood and building boxes

I am very much concerned that some may be underestimating the amount of width shrinkage of saw mill lumber, that has not been air dried or kiln dried. I typically buy a lot of 500 board feet, of saw mill wood. As I strip stack it for drying, I normally take a piece and make the front of a nuc, cut to 9 7/8 inch and date it. Sometimes I measure it less than one year, sometimes I don't. I had several of these laying around the shop from past years, so I took a photo of two recent lots of them to show actual shrinkage. I am in Kentucky, so you likely know weather conditions year around here. It may be different where you live.

My experience over the years is, strip stacked, air dried, pine, cypress, poplar, will shrink 1/2 to 3/4 inch in one year. Almost nothing after that.

In the photo below, the nuc front on the left is poplar. It was cut 9 7/8, on 10 October 2011. It was measured on 5 March 2012 and it measured 9 9/16, In 5 months it has shrunk 5/16, Today, 18 Dec 2012 it is 9 5/16. Total shrinkage in 14 months is 9/16 inch.

The nuc on the right is Pine. It was cut 9 7/8 on 20 December 2011. Today, 18 December 2012 it measured 9 7/16. Shrinkage in 12 months is 7/16 inch. Both were cut from 1 inch green, rough saw, saw mill lumber, planed to 3/4 inch. I never know how long the log has been cut, but, I pick up the wood as soon as the mill has it cut.

I don't know if my experience is normal or not, but, it is what I have experienced and I have the pieces to show how much it shrinks. If someone cut their deep to 9 5/8, then one year later, my experience is that that box would be somewhere near 8 7/8 inch, and would be way too short for a deep super.